I get very sad?

I have two sisters, one older and one younger, and an older brother. They never treat me lke part of the family. When it's just me and one of them, it's okay. But when they're all together, it's like they group up against me. I cry almost everyday because they always ignore me, and they never talk to me. I talked to my mom about this yesterday and she said that everyone is different. When you look at your fingers, they're all different but when you make a fist, they all look the same. But, what I'm trying to say is, is there something I can do to make myself feel more better about myself? I don't want to confront them and talk about it, because they already know how I feel, they just don't care.